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Laurie G. Smith e-mail: lsmith@biomail.ucsd.edu |
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The shapes and positions of plant cells are fixed by cell walls. Consequently, plant cells must be formed during development in the positions where they are needed, requiring exquisite spatial regulation of the cell cycle and cell division planes. Moreover, achievement of appropriate organ and cell shapes depends critically on the orientation of cell expansion. Spatial regulation of cytokinesis and cell expansion are under the control of the cytoskeleton, which provides tracks for intracellular movement, produces or harnesses forces driving intracellular movements, and organizes the cell contents. We are studying cytoskeleton-dependent mechanisms responsible for the spatial control of cytokinesis and cell expansion in plants.
Our studies on the spatial control of cytokinesis have focused primarily on TANGLED, which is implicated by localization and functional studies in division plane control (Figure 1; Walker et al., 2007). We have also studied a functionally redundant pair of PP2A phosphatase subunits, DISCORDIA1 and ALTERNATIVE DISCORDIA1, which are the maize homologs of Arabidopsis FASS/TON2 and are required for PPB formation (Wright et al., 2009). We are now working to further understand the functions of TAN and DCD1/ADD1 with studies in Arabidopsis, maize, and tobacco BY-2 cells.
Figure 1. Schematic of cytokinesis in a somatic plant cell. TANGLED protein co-localizes with the preprophase band (PPB), predicting the future division plane during prophase. Unlike the PPB, TAN remains at the cortical division site throughout mitosis and cytokinesis, where it functions to promote proper placement of the new cell wall. The prerophase/prophase stage is shown in 3-D but other stages are shown in cross section for clarity.
We are also interested in asymmetric cell divisions. In plants, as in other eukaryotes, asymmetric divisions are associated with pattern formation during embryogenesis, establishment of new cell lineages, and the formation of specialized cell types. In all of these processes, developmental asymmetry is closely tied to division polarity, but very little is known about the polarizing cues that orient asymmetric divisions in plants. Our studies in this area have focused on the formation of maize stomata, which form by an invariant sequence of asymmetric divisions (see Figure 2). Recently we discovered a receptor-like protein, PAN1, that is implicated in transmission of a polarizing cue from guard mother cells that orients the asymmetric divisions of adjacent subsidiary mother cells (Cartwright et al., 2009). Future work will focus on elucidating the pathway in which PAN1 functions to polarize cell division in maize along with parallel studies on the closest relatives of PAN1 in Arabidopsis.
Figure 2. Stomatal development in maize. All stages are illustrated schematically except the mature stage, which is illustrated with an image from a Toluidine Blue O-stained epidermal peel. GMC = guard mother cell; SMC = subsidiary mother cell; PPB = preprophase band.
Our studies on the spatial regulation of cell expansion have focused on the role of actin, and have uncovered a role for SCAR and ARP2/3 complex-mediated actin nucleation in the generation of complex cell shapes in the maize and Arabidopsis epidermis (Frank et al., 2002; Djakovic et al., 2006). Most recently, we have shown that WAVE/SCAR subunits are localized at sites of cell growth and wall deposition at the cell surface, suggesting a role for SCAR-ARP2/3-dependent actin polymerization in membrane dynamics at the cell surface (Dyachok et al., 2008).
Cartwright, H.N., Humphries, J.A. and Smith, L.G. (2009). PAN1: A receptor-like protein that promotes polarization of an asymmetric division in maize. Science 323:649-651.
Djakovic, S.N., Dyachok, J., Burke, M.P., Frank, M.J., and Smith, L.G. (2006). BRICK1/HSPC300 acts with SCAR and the ARP2/3 complex to regulate epidermal cell shape in Arabidopsis. Development 133:1091-1100.
Dyachok, J., Shao, M.-R., Vaughn, K., Bowling, A., Facette, M., Djakovic, S., Clark, L., and Smith, L.G. (2008). Plasma membrane-associated SCAR complex subunits promote cortical F-actin accumulation and normal growth characteristics in Arabidopsis roots. Molecular Plant 1:990-1006.
Frank, M.J., and Smith, L.G. (2002). A small, novel protein highly conserved in plants and animals promotes the polarized growth and division of maize leaf epidermal cells. Curr. Biol 12:849-853.
Frank, M., Egile, C., Dyachok, J., Djakovic, S., Nolasco, M., Li, R., and Smith, L.G. (2004). Activation of Arp2/3 complex-dependent actin polymerization by plant proteins distantly related to Scar/WAVE. Proc. Nat. Acad. Sci. USA 101:16379-16384.
Muller, S., Han, S., and Smith, L.G. (2006). Two kinesins are involved in the spatial control of cytokinesis in Arabidopsis thaliana. Curr. Biol., 16:888-894.
Walker, K.L., Müller, S., Moss, D., Ehrhardt, D.W., and Smith, L.G. (2007). Arabidopsis TANGLED identifies the division plane throughout mitosis and cytokinesis. Curr. Biol., 17:1827-1836.
Wright, A.J., Gallagher, K., and Smith, L.G. (2009). DCD1 and ADD1 function redundantly at the cortical division site to promote PPB formation and orient division planes in maize. The Plant Cell, in press.
After receiving her Ph.D. from University College, London, Laurie Smith conducted postdoctoral study at Stanford University and the UC Berkeley/USDA Plant Gene Expression Center.
